Microstrip line filter

ABSTRACT

Disclosed herein is a microstrip line filter including a dielectric substrate, a first hairpin resonator, and a metal ground layer. The first hairpin resonator is disposed on a first layer of the dielectric substrate. The metal ground layer is disposed on a second layer of the dielectric substrate. The metal ground layer includes a first defected ground structure. The first defected ground structure includes a first defected area, a second defected area, and a third defected area. The projection of the first defected area on the first layer is located inside the hairpin structure of the first hairpin resonator. The projection of the second defected area on the first layer is located in a direction opposite to an opening direction of the first hairpin resonator. The third defected area connects the first defected area and the second defected area.

This application claims the benefits of People's Republic of Chinaapplication Serial No. 201410326631.2, filed Jul. 10, 2014, and SerialNo. 201510175623.7, filed Apr. 14, 2015, the subject matters of whichare incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates in general to a microstrip line filter, and moreparticularly to a microstrip line filter having a defected groundstructure (DGS).

2. Description of the Related Art

As technology advances, wireless communication technologies, such asWi-Fi, ZigBee, and Bluetooth, have been widely used in people's everydaylife. Filter is an important component in wireless communicationproducts because the filter is capable of removing unwanted frequencycomponents from signal, which improves signal quality of wirelesscommunication products.

In response to a trend of small and lightweight communication productsand the user's request for a higher communication quality, there is aneed for reducing the area occupied by the filter, decreasing themanufacturing cost, and providing the filter with better frequencycharacteristics.

SUMMARY OF THE INVENTION

The invention is directed to a microstrip line filter having betterfrequency characteristics and requiring smaller circuit area.

According to one embodiment of the present invention, a microstrip linefilter is disclosed. The microstrip line filter includes a dielectricsubstrate, a first hairpin resonator and a metal ground layer. The firsthairpin resonator is disposed on a first layer of the dielectricsubstrate. The metal ground layer is disposed on a second layer of thedielectric substrate, and includes a first defected ground structure.The first defected ground structure includes a first defected area, asecond defected area and a third defected area. The projection of thefirst defected area on the first layer is located inside the hairpinstructure of the first hairpin resonator. The projection of the seconddefected area on the first layer is located in a direction opposite toan opening direction of the first hairpin resonator. The third defectedarea connects the first defected area and the second defected area.

The microstrip line filter provided in this disclosure achieves betterfrequency characteristics because the microstrip circuit on the printedcircuit board is accompanied with a corresponding defected groundstructure. Furthermore, because the defected ground structure may bealigned with the microstrip line circuit disposed on the printed circuitboard, equivalent inductance can be increased and hence the samefiltering function can be accomplished with a smaller circuit area.

The above and other aspects of the invention will become betterunderstood with regard to the following detailed description of thepreferred but non-limiting embodiment(s). The following description ismade with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a schematic diagram of a microstrip line filter accordingto the first embodiment of the invention;

FIG. 1B shows a top view of the microstrip line filter according to thefirst embodiment of the invention;

FIG. 1C shows a bottom view of the metal ground layer of the microstripline filter according to the first embodiment of the invention;

FIG. 2 shows a top view of a microstrip line filter according to thesecond embodiment of the invention;

FIG. 3 shows a diagram of frequency response obtained from measurementaccording to the second embodiment of the invention;

FIG. 4 shows a schematic illustrating an equivalent circuit of ahigh-pass filter in a microstrip line filter according to the thirdembodiment of the invention;

FIG. 5 shows a diagram of frequency response obtained from measurementaccording to the third embodiment of the invention;

FIG. 6A shows a top view of a microstrip line filter according to thefourth embodiment of the invention;

FIG. 6B shows a bottom view of a microstrip line filter according to thefourth embodiment of the invention;

FIG. 7A and FIG. 7B show diagrams of frequency response obtained frommeasurement according to the fourth embodiment of the invention;

FIG. 8A shows a top view of a microstrip line filter according to thefifth embodiment of the invention;

FIG. 8B shows a bottom view of a microstrip line filter according to thefifth embodiment of the invention;

FIG. 9A shows a top view of a microstrip line filter according to thesixth embodiment of the invention;

FIG. 9B shows a bottom view of a microstrip line filter according to thesixth embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

A number of embodiments and accompanying drawings are disclosed belowfor elaborating the invention, not for limiting the scope of protectionof the invention.

According to a conventional approach, a filter, such as a low-passfilter, a high-pass filter or a band-pass filter, is formed by lumpedelements including resistors, capacitors and inductors. For a filterconsisting of lumped elements, in order to achieve greater attenuationon the second harmonic and the third harmonic frequency components, morelumped elements are required. However, more lumped elements occupy alarger circuit area and increase manufacturing cost, which limitspossible applications of the filter.

According to the embodiments of the invention, the filter may be formedby microstrip line circuits on a printed circuit board (PCB). Theperformance of the filter thus formed is better than that of the filterformed by lumped elements in particular regarding the high-frequencyportion of the frequency response (the lumped elements generate aparasitic effect at high frequencies). Since the filter may be directlyformed on a printed circuit board, there is no need to purchaseadditional elements and the manufacturing cost is effectively reduced.

In order to achieve better frequency characteristics, for example,better attenuation effect on signals at the stop-band, defectedstructures are created in the ground plane of a printed circuit board toform a specific pattern. For example, a defected ground structure isformed by removing a portion of the metal on the ground plane by anetching process. The defected ground structure can form equivalentinductors and capacitors, and thus change current distribution andachieve better filter frequency characteristics. Since the filter inthis disclosure is formed by microstrip line circuits on PCB instead oflumped elements, the filter can be manufactured consistently and haslittle variation in frequency characteristics. Detailed descriptions ofthe embodiments of the invention are disclosed below.

FIG. 1A shows a schematic diagram of a microstrip line filter 1according to the first embodiment of the invention. The microstrip linefilter 1 includes a dielectric substrate 10, a first hairpin resonator11 and a metal ground layer 12. To more clearly illustrate the firstembodiment of the invention, FIGS. 1B and 10 are provided. FIG. 1B showsa top view of the microstrip line filter 1 according to the firstembodiment of the invention. FIG. 1C shows a bottom view of the metalground layer 12 of the microstrip line filter 1 according to the firstembodiment of the invention.

The first hairpin resonator 11 is disposed on a first layer of thedielectric substrate 10, such as an upper surface of the dielectricsubstrate 10. The metal ground layer 12 is disposed on a second layer ofthe dielectric substrate 10. The metal ground layer 12 includes a firstdefected ground structure 13. The first defected ground structure 13includes a first defected area 131, a second defected area 132 and athird defected area 133. The projection of the first defected area 131on the first layer of the dielectric substrate 10 is located inside thehairpin structure of the first hairpin resonator 11. The projection ofthe second defected area 132 on the first layer of the dielectricsubstrate 10 is located in a direction opposite to an opening directionof the first hairpin resonator 11. The third defected area 133 connectsthe first defected area 131 and the second defected area 132. Detaileddescriptions of each element are disclosed below.

Referring to FIG. 1B, the first hairpin resonator 11 includes a firstmetal strip 111, a second metal strip 112, and a U-shaped metal strip113. The second metal strip 112 is parallel to and separated from thefirst metal strip 111 by a distance d. One end of the U-shaped metalstrip 113 is connected to the first metal strip 111, and the other endof the U-shaped metal strip 113 is connected to the second metal strip112. Therefore, the U-shaped metal strip 113 in conjunction with thefirst metal strip 111 and the second metal strip 112 forms a hairpinstructure having an opening. As shown in FIG. 1B, the opening directionof the first hairpin resonator 11 is the direction D1.

The U-shaped metal strip 113 used in the present embodiment includes afirst metal segment 1131, a second metal segment 1132 and a third metalsegment 1133. The first metal segment 1131 is connected to the firstmetal strip 111. The second metal segment 1132 is perpendicular to thefirst metal segment 1131 and the third metal segment 1133. The thirdmetal segment 1133 is connected to the second metal strip 112.

In the first hairpin resonator 11, both the first metal strip 111 andthe first metal segment 1131 connected to the first metal strip 111 arerectangular; the line width of the first metal segment 1131 is notequivalent to that of the first metal strip 111. The line width LW3 ofthe first metal segment 1131 is smaller than the line width LW1 of thefirst metal strip 111. The third metal segment 1133 and the first metalsegment 1131 have the same shape and the same size. The second metalstrip 112 and the first metal strip 111 have the same shape and the samesize (the first metal strip 111 and the second metal strip 112 formparallel coupled microstrip lines). Therefore, the first hairpinresonator 11 is a symmetric structure. The line width LW4 of the thirdmetal segment 1133 is smaller than the line width LW2 of the secondmetal strip 112.

Descriptions of the first defected ground structure 13 of the presentembodiment are made with reference to FIG. 10. FIG. 10 illustrates alower surface of the dielectric substrate 10. The metal ground layer 12is disposed on the lower surface of the dielectric substrate 10, shownas shaded area in FIG. 10. An etching process is performed on the metalground layer 12 to remove a specific portion of metal to form the firstdefected ground structure 13. In FIG. 10, the first defected area 131,the second defected area 132 and the third defected area 133 are blankareas representing the areas where the metal has already been etched.

In the specification, the descriptions regarding the relative positionbetween the first defected ground structure 13 and the first hairpinresonator 11 all refer to the projection of the first defected groundstructure 13 on the upper surface of the dielectric substrate 10. Tomake the specification more concise, the term “projection” will not berepeatedly hereinafter. The first defected area 131 is located insidethe first hairpin resonator 11. In the present embodiment, the firstdefected area 131 is surrounded by the U-shaped metal strip 113. Thesecond defected area 132 is located in a direction opposite to theopening direction of the first hairpin resonator 11, that is, anopposite direction of direction D1, and located outside the hairpinstructure of the first hairpin resonator 11. The third defected area133, which connects the first defected area 131 and the second defectedarea 132, crosses the first hairpin resonator 11.

In the present embodiment, the first defected area 131 is rectangular.The second defected area 132 is rectangular, and has the same size asthat of the first defected area 131. The third defected area 133 isrectangular. The third defected area 133 is across and perpendicular tothe second metal segment 1132 of the U-shaped metal strip 113.Therefore, the first defected ground structure 13 has a dumbbell shape.One end of the dumbbell is located inside the hairpin structure of thefirst hairpin resonator 11, and the other end of the dumbbell is locatedoutside the hairpin structure of the first hairpin resonator 11. Thecenter portion of the dumbbell is across and perpendicular to the secondmetal segment 1132 and passes through the middle point of the secondmetal segment 1132.

The microstrip line filter 1 formed by the first hairpin resonator 11and the first defected ground structure 13 is a low-pass filter capableof removing high-frequency components of the signal. The microstrip linefilter 1 includes a signal feed point 14 and a signal output point 15.The signal feed point 14 is near the one end of the U-shaped metal strip113 connected to the first metal strip 111. The signal output point 15is near the other end of the U-shaped metal strip 113 connected to thesecond metal strip 112. The frequency response of the microstrip linefilter 1 may be changed by adjusting the sizes of the first metal strip111, the second metal strip 112, the U-shaped metal strip 113, the firstdefected area 131, the second defected area 132, and the third defectedarea 133. For example, parameters related to the frequency response maybe changed, including cutoff frequency, return loss, insertion loss, andattenuation on the second and third harmonics.

The present embodiment discloses the shapes of the first hairpinresonator 11 and the first defected ground structure 13. However, aperson skilled in the art would understand that the shapes of the firsthairpin resonator 11 and the first defected ground structure 13 are notlimited to the examples disclosed in the present embodiment, and may bedesigned according to the application and the desired frequencyresponse. A number of other possible implementations are disclosedbelow. Any design in which the first defected area 131 is located insidethe hairpin structure of the first hairpin resonator 11, the seconddefected area 132 is located in a direction opposite to the openingdirection of the first hairpin resonator 11, and the third defected area133 connects the first defected area 131 and the second defected area132 is within the scope of the invention.

The first defected ground structure 13 may be S-shaped. For example, thefirst defected area 131 is rectangular, the second defected area 132 isC-shaped, one end of the second defected area 132 is connected to thethird defected area 133, and the third defected area 133 is connected toa corner of the first defected area 131. The S-shaped defected groundstructure is illustrated in FIG. 6B.

The microstrip line filter of the disclosed embodiments has a defectedground structure, which effectively reduces the size of the microstripline circuit on the upper layer of the dielectric substrate andaccordingly reduces the area required by the filter. Specifically, toachieve the same cutoff frequency, the U-shaped metal strip 113 used ina microstrip line filter having a defected ground structure can beshorter than that used in a microstrip line filter without a defectedground structure. In addition, the defected ground structure alsoimproves the frequency characteristics of the microstrip line filter,such that the filter has a greater attenuation at the stop-band.Frequency response of a conventional first order filter has a slope of−6 dB per octave. If a larger attenuation rate is desired, the filtermust be a higher order filter, which requires a larger circuit area. Themicrostrip line filter having a defected ground structure as disclosedherein requires a small circuit area and achieves good filter frequencycharacteristics.

The microstrip line filter 1 of the first embodiment includes a firsthairpin resonator 11 and a first defected ground structure 13. Toachieve better filter frequency response, one more stage of filter maybe connected to the signal output end 15 of the first embodiment.Detailed descriptions of the second embodiment of the invention aredisclosed below.

FIG. 2 shows a top view of a microstrip line filter according to thesecond embodiment of the invention. Like the first embodiment, a metalground layer 22 is disposed on a lower surface of the dielectricsubstrate, and the details are not repeated here. The microstrip linefilter 2 of the present embodiment further includes a second hairpinresonator 24 and a connecting metal strip 25 in addition to the firsthairpin resonator 21 and the first defected ground structure 23 that arealready disclosed in the first embodiment. The opening direction of thesecond hairpin resonator 24 is the same as the opening direction of thefirst hairpin resonator 21, which is the direction D2 shown in FIG. 2.The connecting metal strip 25 connects the first hairpin resonator 21and the second hairpin resonator 24. The metal ground layer 22 furtherincludes a second defected ground structure 26.

In the embodiment illustrated in FIG. 2, two identical filters areconnected in cascade. However, any person skilled in the art wouldunderstand that the two stages of filters may have the same or differentstructures. That is, the second hairpin resonator 24 and the firsthairpin resonator 21 may have different shapes, and the second defectedground structure 26 and the first defected ground structure 23 also mayhave different shapes. Furthermore, the two stages of filters may havesimilar structures with different sizes. Any structures and sizes woulddo as long as the microstrip line filters connected in cascade canachieve the desired frequency response.

FIG. 3 shows a diagram of frequency response obtained from measurementaccording to the second embodiment of the invention. The measurement isbased on the microstrip line filter 2 with a width W2 of 6.6 mm and aheight H2 of 6.9 mm, wherein the size is selected according to thefrequency band requirement of the wireless communication product used inactual applications. In FIG. 3, curve L1 represents a return loss of themicrostrip line filter 2, that is, the parameter S11. Normally, thereturn loss has a threshold value of −10 dB. As shown in FIG. 3, themicrostrip line filter 2 has a good return loss when the frequency isaround 2.4 GHz. Curve L2 represents an insertion loss of the microstripline filter 2, that is, the parameter S21. Curve L2 shows that themicrostrip line filter 2 is a low-pass filter, and the cutoff frequencyis about 2.6 GHz. Therefore, the microstrip line filter 2 of the presentembodiment can be used in wireless communication products having afrequency band of 2.4 GHz, such as the wireless communication productsusing Wi-Fi or ZigBee radios. The filter has an insertion loss of −1.3dB at the frequency of 2.4 GHz. Note that the filter has an insertionloss of −68.25 dB at the frequency corresponding to the second harmonic(that is, 4.8 GHz). In comparison to the conventional filter, themicrostrip line filter 2 greatly increases the amount of attenuation onthe second harmonic.

The filter parameters may be adjusted according to the requiredspecifications. For example, the design of the filter can be adjustedaccording to the insertion loss at frequency of 2.4 GHz, the insertionloss at frequency of the second harmonic, and the insertion loss atfrequency of the third harmonic. Table 1 shows actually measuredfrequency response of the low-pass filter designed according todifferent needs.

TABLE 1 S11 (dB) S21 (dB) S21 (dB) S21 (dB) Filter (2.4 GHz) (2.4 GHz)(4.8 GHz) (7.2 GHz) Example 1 −16.223 −1.02 −73.179 −60.939 Example 2−17.249 −0.633 −36.281 −78.445

As shown in Table 1, the filter proposed in this disclosure has goodattenuation on the second harmonic and the third harmonic. The returnloss can also fulfill actual needs. No matter the filter is used in atransmitting end or in a receiving end of wireless signals, signaldistortion and interference will be greatly reduced if the harmoniccomponents can be attenuated by a large amount. For example, thetransmitter can avoid transmitting the harmonics generated by internalcircuits, and the receiver can filter out the harmonic components of thesignal when receiving a signal.

The microstrip line filter 2 of the second embodiment is a low-passfilter. To realize a band-pass filter, a high-pass filter may be cascadeconnected to the output end or the input end of the microstrip linefilter 2. Therefore, the microstrip line filter of the third embodimentof the invention is realized by cascade connecting a high-pass filter 4to the second hairpin resonator 24 of the second embodiment, wherein thehigh-pass filter 4 may be formed by lumped elements. FIG. 4 shows aschematic illustrating an equivalent circuit of a high-pass filter of amicrostrip line filter according to the third embodiment of theinvention. The high-pass filter 4 includes an input terminal P1, anoutput terminal P2, a capacitor Cx, and inductors Lx and Ly.

FIG. 5 shows a diagram of frequency response obtained from measurementaccording to the third embodiment of the invention. Curve L3 representsa return loss of the microstrip line filter of the third embodiment. Asshown in FIG. 5, the microstrip line filter still has a good return lossat frequency of 2.4 GHz. Curve L4 represents an insertion loss of themicrostrip line filter of the third embodiment. It can be seen fromcurve L4 that the microstrip line filter is a band-pass filter, and onlythe frequency components near 2.4 GHz are allowed to pass. Therefore,the components of the signal with a lower frequency, such as 900 MHz,cannot pass through the band-pass filter (having an insertion loss of−25.13 dB). The filter has an insertion loss of −58.03 dB at thefrequency of the second harmonic (4.8 GHz), and has an insertion loss of−53.00 dB at the frequency of the third harmonic (7.2 GHz). Therefore,the filter of the third embodiment can be used in wireless communicationproducts having a frequency-band of 2.4 GHz, and has good attenuation onharmonic components.

FIG. 6A shows a top view of a microstrip line filter according to thefourth embodiment of the invention. FIG. 6B shows a bottom view of amicrostrip line filter according to the fourth embodiment of theinvention. Like previous embodiments, the filter of the fourthembodiment may also be realized by connecting two stages of identicalfilters in cascade, and thus the details are not repeated here. However,the defected ground structures used in the microstrip line filter 5 ofthe fourth embodiment may have different shapes from those in previousembodiments.

The microstrip line filter 5 includes a first hairpin resonator 51, asecond hairpin resonator 54 and a connecting metal strip 55. The metalground layer 52 includes a first defected ground structure 53 and asecond defected ground structure 56. Since the second defected groundstructure 56 and the first defected ground structure 53 havesubstantially the same structure, only the first defected groundstructure 53 is described below.

Referring to FIG. 6B, the first defected ground structure 53 includes afirst defected area 531, a second defected area 532 and a third defectedarea 533. In the present embodiment, the first defected area 531 isrectangular and located inside the hairpin structure of the firsthairpin resonator 51. Note that the relative position of the firstdefected area 531 is different from that of the first defected area 131shown in FIG. 10 where the first defected area 131 is completelysurrounded by the U-shaped metal strip 113. The second defected area 532is C-shaped, that is, the second defected area 532 has an opening, andthe opening direction D3 of the C-shaped second defected area 532 isperpendicular to the opening direction D4 of the first hairpin resonator51. The second defected area 532 is located in a direction opposite tothe opening direction of the first hairpin resonator 51. The thirddefected area 533 is rectangular and connected to a corner of therectangular first defected area 531. One end of the C-shaped seconddefected area 532 is connected to the third defected area 533. As awhole, the first defected ground structure 53 is S-shaped.

FIG. 7A and FIG. 7B show diagrams of frequency response obtained frommeasurement according to the fourth embodiment of the invention. Themeasurement is based on the microstrip line filter 5 with a width W5 of11.6 mm and a height H5 of 5.8 mm. FIG. 7A shows the relation betweeninsertion loss and frequency. The target frequency band of themicrostrip line filter 5 is 2.45 GHz. As shown in FIG. 7A, the filterhas an insertion loss of −0.45 dB at the frequency of 2.45 GHz, aninsertion loss of −46.94 dB at the frequency of the second harmonic (4.9GHz), and an insertion loss of −39.19 dB at frequency of the thirdharmonic (7.35 GHz). Therefore, the microstrip line filter 5 effectivelyattenuates harmonic components of the signal. FIG. 7B shows the relationbetween return loss and frequency. The filter has a return loss of−10.08 dB at the frequency of 2.45 GHz. Therefore, the microstrip linefilter 5 can be used in wireless communication products operating at afrequency band of 2.45 GHz.

The microstrip line filters disclosed in the above embodiments areformed by microstrip line circuits on a printed circuit board. Aspecific pattern is formed on the metal ground layer of the printedcircuit board to form the defected ground structure. The filter formedby the defected ground structure along with the microstrip line circuiton the upper surface of the printed circuit board achieves goodfrequency characteristics, particularly in attenuating harmoniccomponents. Moreover, the filter may be formed directly on the printedcircuit board, and hence additional lumped element or additional specialmanufacturing process on the printed circuit board is not required. Thefilter thus formed not only occupies a smaller circuit area but alsoreduces the manufacturing cost. Although the filters in the aboveembodiments have a pass band around 2.45 GHz, the filter proposed inthis disclosure may be adapted to any frequency band, for example, apass band around 5 GHz may also be applicable.

FIG. 8A shows a top view of a microstrip line filter according to thefifth embodiment of the invention. FIG. 8B shows a bottom view of amicrostrip line filter according to the fifth embodiment of theinvention. The microstrip line filter 6 of the present embodiment isdifferent from the microstrip line filter 1 of FIGS. 1A-1C in therelative relation between the hairpin resonator 61 and the defectedground structure 63. Like the first embodiment, the defected groundstructure 63 includes a first defected area 631, a second defected area632 and a third defected area 633. The first defected area 631 islocated inside the hairpin structure of hairpin resonator 61. The seconddefected area 632 is located in a direction opposite to the openingdirection of the hairpin resonator 61. The third defected area 633connects the first defected area 631 and the second defected area 632.

As shown in FIG. 8A, the hairpin resonator 61 has a first edge 6101 anda second edge 6102 disposed opposite to each other inside the hairpinstructure of the hairpin resonator 61. As shown in FIG. 8B, the firstdefected area 631 has at least one third edge 6313. Please refer to FIG.8A and FIG. 8B, the projection of the third edge 6313 on the first layerof the dielectric substrate is aligned with the first edge 6101.

As shown in FIG. 8B, the first defected area 631 may further have atleast one fourth edge 6314 parallel to the third edge 6313 and alignedwith the second edge 6102. Note that the first defected area 631 mayalso be other shapes. In this exemplary embodiment the third edge 6313is parallel to the fourth edge 6314.

The microstrip line filter 6 may be used as a radio frequency filter (RFfilter) capable of transmitting alternating current signals. When ametal conductor transmits alternating current signals, the currentdensity is largest near the surface of the conductor and decreases withgreater depths in the conductor due to the skin effect. In FIG. 8A, thepart of the metal conductor near the first edge 6101 and the second edge6102 has a larger current density. In FIG. 8B, the part of the metalground layer 62 near the third edge 6313 and the fourth edge 6314 has alarger current density.

For the alternating current signals, at a particular time instant,assume the current I1 on the first edge 6101 flows upwards, meanwhile,the current I2 on the second edge 6102 flows downwards, the current I3of the metal ground layer 62 near the third edge 6313 flows upwards, andthe current I4 of the metal ground layer 62 near the fourth edge 6314flows downwards. Current generates an induced magnetic field. Therefore,when the current on the upper surface and the current on the lowersurface flow in the same direction at the place where metal edges arealigned, for example, the current I1 and the current I3 flow in the samedirection, the direction of the induced magnetic field generated by thecurrent I1 will be the same as that generated by the current I3. Themagnetic lines of force generated by the current I1 superimposed on themagnetic lines of force with substantially the same direction generatedby the current I3 increases the mutual inductance.

Therefore, when the first edge 6101 is aligned with the third edge 6313,equivalent inductance will be increased. Likewise, when the second edge6102 is aligned with the fourth edge 6314, equivalent inductance will beincreased as well. Since the equivalent inductance is increased when thestructure on the upper surface is aligned with the structure on thelower surface, the same inductance value can be achieved with a smallercircuit area. That is, the circuit area can be effectively reduced, andthe same frequency response can still be achieved.

FIG. 9A shows a top view of a microstrip line filter according to thesixth embodiment of the invention. FIG. 9B shows a bottom view of amicrostrip line filter according to the sixth embodiment of theinvention. The microstrip line filter 7 of the present embodimentdiffers from the microstrip line filter 6 in the shape of the defectedground structure 73. The first defected area 731 may include multiplefirst defected segments 7310 perpendicular to each other. The seconddefected area 732 may include multiple second defected segments 7320perpendicular to each other. Both the first defected area 731 and thesecond defected area 732 may be S-shaped defected ground structures.However, the defected ground structure is not limited to a strictS-shaped structure and may also include more bending points. Themicrostrip line filter 7 has aligned structure between the upper surfaceand the lower surface.

As shown in FIG. 9A, the hairpin resonator 71 has a first edge 7101 anda second edge 7102 opposite to each other inside the hairpin structureof first hairpin resonator 71. As shown in FIG. 9B, the first defectedarea 731 has multiple third edges 7313 arranged in the same direction.In this example, multiple third edges 7313 are located at the right-handside of the first defected area 731, and the first edge 7101 is alignedwith multiple third edges 7313. The first defected area 731 may furtherhave multiple fourth edges 7314 arranged in the same direction. In thisexample, multiple fourth edges 7314 are located at the left-hand side ofthe first defected area 731. Multiple third edges 7313 are parallel tomultiple fourth edges 7314, and the second edge 7102 is aligned withmultiple fourth edges 7314.

Like the previous embodiment, at a particular time instant, assume thecurrent I5 on the first edge 7101 flows upwards, meanwhile, the currentI6 on the second edge 7102 flows downwards, the current I7 on the metalground layer 72 near multiple third edges 7313 flows upwards, and thecurrent I8 of the metal ground layer 72 near multiple fourth edges 7314flows downwards. Since the current on the upper surface and the currenton the lower surface have the same direction at the place where metaledges are aligned, the equivalent inductance is increased. The presentembodiment discloses a defected ground structure whose shape isdifferent from that disclosed in previous embodiments. The filter of thepresent embodiment increases the equivalent inductance and thus requiressmaller circuit area.

In the microstrip line filter 5 of FIG. 6A and FIG. 6B, the structure onthe upper surface and the structure on the lower surface of thedielectric substrate may also be aligned. For example, an edge insidethe hairpin structure of the first hairpin resonator 51 may be alignedwith an edge of the first defected area 531. The inductance may also beincreased in this example.

According to the microstrip line filters of the above embodiments, adefected ground structure is combined with a microstrip line circuitdisposed on the upper surface of a printed circuit board, such that thefilter achieves good frequency characteristics. Furthermore, because thedefected ground structure is aligned with the microstrip line circuitdisposed on the upper surface of the printed circuit board, theequivalent inductance can be increased, and hence the same filterperformance can be accomplished with smaller circuit area.

While the invention has been described by way of example and in terms ofthe preferred embodiment(s), it is to be understood that the inventionis not limited thereto. On the contrary, it is intended to cover variousmodifications and similar arrangements and procedures, and the scope ofthe appended claims therefore should be accorded the broadestinterpretation so as to encompass all such modifications and similararrangements and procedures.

What is claimed is:
 1. A microstrip line filter, comprising: adielectric substrate; a first hairpin resonator, disposed on a firstlayer of the dielectric substrate; and a metal ground layer, disposed ona second layer of the dielectric substrate, wherein the metal groundlayer comprises a first defected ground structure, the first defectedground structure comprising: a first defected area, wherein projectionof the first defected area on the first layer is located inside ahairpin structure of the first hairpin resonator; a second defectedarea, wherein projection of the second defected area on the first layeris located in a direction opposite to an opening direction of the firsthairpin resonator; and a third defected area, connecting the firstdefected area and the second defected area.
 2. The microstrip linefilter according to claim 1, wherein the first defected area isrectangular, the second defected area is rectangular, and the seconddefected area and the first defected area have the same size.
 3. Themicrostrip line filter according to claim 1, wherein the first defectedarea is rectangular, the second defected area is C-shaped, one end ofthe second defected area is connected to the third defected area, andthe third defected area is connected to a corner of the first defectedarea.
 4. The microstrip line filter according to claim 1, wherein thefirst hairpin resonator comprises: a first metal strip; a second metalstrip parallel to the first metal strip; and a U-shaped metal strip,wherein one end of the U-shaped metal strip is connected to the firstmetal strip, and the other end of the U-shaped metal strip is connectedto the second metal strip.
 5. The microstrip line filter according toclaim 4, wherein the U-shaped metal strip comprises: a first metalsegment, connected to the first metal strip; a second metal segment,perpendicular to the first metal segment; and a third metal segment,perpendicular to the second metal segment and connected to the secondmetal strip; wherein the third defected area is rectangular, andprojection of the third defected area on the first layer isperpendicular to the second metal segment of the U-shaped metal strip.6. The microstrip line filter according to claim 4, wherein the U-shapedmetal strip comprises: a first metal segment, connected to the firstmetal strip; a second metal segment, perpendicular to the first metalsegment; and a third metal segment, perpendicular to the second metalsegment and connected to the second metal strip; wherein the first metalstrip is rectangular, the first metal segment of the U-shaped metalstrip is rectangular, a line width of the first metal segment is smallerthan a line width of the first metal strip, the third metal segment andthe first metal segment have the same shape and the same size, and thesecond metal strip and the first metal strip have the same shape and thesame size.
 7. The microstrip line filter according to claim 4, furthercomprising a signal feed point near the one end of the U-shaped metalstrip connected to the first metal strip.
 8. The microstrip line filteraccording to claim 1, further comprising: a second hairpin resonator,wherein an opening direction of the second hairpin resonator is the sameas the opening direction of the first hairpin resonator; and aconnecting metal strip, connecting the first hairpin resonator and thesecond hairpin resonator; wherein the metal ground layer furthercomprises a second defected ground structure whose projection on thefirst layer overlaps the second hairpin resonator.
 9. The microstripline filter according to claim 8, wherein the second hairpin resonatorand the first hairpin resonator have the same shape and the same size,the second defected ground structure and the first defected groundstructure have the same shape and the same size, relative positionbetween the second defected ground structure and the second hairpinresonator is the same as relative position between the first defectedground structure and the first hairpin resonator.
 10. The microstripline filter according to claim 8, further comprising a high-pass filterconnected to the second hairpin resonator.
 11. The microstrip linefilter according to claim 1, wherein the first hairpin resonator has afirst edge and a second edge opposite to each other inside the hairpinstructure of the first hairpin resonator, and the first defected areahas at least one third edge aligned with the first edge.
 12. Themicrostrip line filter according to claim 11, wherein the first defectedarea further has at least one fourth edge parallel to the at least onethird edge and aligned with the second edge.
 13. The microstrip linefilter according to claim 11, wherein the first defected area comprisesa plurality of first defected segments perpendicular to each other. 14.The microstrip line filter according to claim 11, wherein the seconddefected area comprises a plurality of second defected segmentsperpendicular to each other.